MGS MOC Release No. MOC2-1223, 20 September 2005

One of the most profound discoveries that would not have
been possible if the Mars Global Surveyor (MGS) mission
had not been extended beyond its 1 Mars year (687 Earth days)
Primary Mission is that of dramatic changes that take place
in the south polar residual ice cap each martian year. To make
this discovery, the MGS Mars Orbiter Camera (MOC) had to be
employed during a second Mars year to repeat images of sites
on the south polar cap that had been imaged during the Primary
Mission.

The initial discovery was made in 2001, when the MOC team repeated
images of portions of the south polar cap that had already been
imaged in 1999. The goal of these images was to obtain stereo
views, which would allow investigators to see the topography of
the cap in three dimensions (3-D) and to measure the thickness of
the polar ice layers.

It was not possible to produce the desired 3-D views. To the
team's surprise, the landforms of the south polar cap had changed.

The south polar residual cap -- that is, the portion of the ice cap that
remains bright and retains ice throughout the southern summer
season -- was seen in 1997 and 1999 MOC images to have a complex terrain
of broad, relatively flat mesas, small buttes, and many pits and
troughs. Pits are generally circular and in some areas visually
resemble a stack of thin slices of Swiss cheese. Very early in the MGS
mission, the MOC team speculated that these landforms must be carved
into frozen CO2, because they look so unfamiliar and
because Viking orbiter infrared measurements indicated that the south
polar cap is cold enough consist of frozen carbon dioxide, even in summer.

The observations made by MOC in 2001, during the first part of
the MGS Extended Mission, showed that the scarps and pit walls
of the south polar cap had retreated at an average rate of about
3 meters (~10 feet) since 1999. In other words, 3 meters per Mars year
(and, of course, most of that retreat takes place during the summer).
In some places on the cap, the scarps will retreat less than 3 meters
a Mars year, and in others it can retreat as much as 8 meters per
martian year.

Of the two materials that one is likely to encounter in a frozen
state on Mars -- water and carbon dioxide -- it is CO2 that
is volatile enough to permit scarp retreat rates like those observed
by MOC.

Over time, south polar pits merge to become plains, mesas turn into
buttes, and buttes vanish forever. Since 2001, two additional Mars
years have elapsed. A scientific benefit of having a long MGS Extended
Mission has been the opportunity to use MOC to document how the polar cap
is changing each year.

Four images are shown here. Each is located near 86.3°S, 49.4°W,
and each shows the same portion of the south polar residual cap as
it appeared in 1999, 2001, 2003, and 2005. The animated GIF image
shows a "movie" recording the changes that have occurred during the
past four martian years. The landscape of the south polar cap is
changing very rapidly.

Each year that MGS has been in orbit, the landforms of the south polar
residual cap have gotten smaller, and the CO2 removed from
the cap has not been re-deposited. The implication is that Mars
presently has a warm (and warming?) climate, with new carbon dioxide
going into the atmosphere every year. The other implication is that,
at some time in the not-too-distant past, the planet had a colder
climate, so that the layers of CO2 could be deposited in
the first place. If one takes the rate of scarp retreat and projects
it backwards to fill in all of the pits and troughs with the carbon
dioxide that has been removed from them, one finds that the colder
climate might only have occurred a few centuries to a few tens of
thousands of years ago. This kind of time scale is not unlike that of
the climate changes that have been recorded on Earth, including the
Ice Ages and the smaller fluctuations that have occurred since the
last Ice Age (e.g., the "Little Ice Age" of the mid-14th
through mid-19th centuries).

After the discovery that the pits were enlarging and that we were not
seeing CO2 deposition, it was suggested that interannual
variations might be large enough to permit such deposition on a short
timescale. However, two Mars years of additional observations show no
large magnitude annual differences. Variations that would permit
CO2 deposition may require decades. And to see such
variations may require many more Mars years of observations by
orbiting spacecraft.

Malin Space Science Systems and the California Institute of Technology
built the MOC using spare hardware from the Mars Observer mission.
MSSS operates the camera from its facilities in San Diego, California.
The Jet Propulsion Laboratory's Mars Surveyor Operations Project
operates the Mars Global Surveyor spacecraft with its industrial
partner, Lockheed Martin Astronautics, from facilities in Pasadena,
California and Denver, Colorado.